Digital amplifier with feedforward and feedback control

ABSTRACT

The invention relates to a digital amplifier for providing a desired electrical output power, the amplifier comprising a power source ( 100 ) for generating the electrical output power, the amplifier further comprising: a digital input adapted for receiving a digital input signal ( 112 ), the digital input signal ( 112 ) representing the desired electrical output power level, a reference power generator ( 124 ) for generating an analogue reference power controlled by the digital input signal ( 112 ), a power measurement component ( 142; 128 ) adapted for measuring the power differential between the electrical output power provided by the power source ( 100 ) and the analogue reference power, an analogue-to-digital converter ( 130 ) adapted for converting the power differential into a digital power differential value ( 132 ), a combiner adapted for providing a combined digital signal ( 136 ) by adding the digital power differential value ( 132 ) to the digital value input to the reference power generator ( 124 ) for generating the analogue reference power, wherein the power source ( 100 ) is adapted for providing the electrical power corrected for the difference between the power indicated by the digital input signal ( 112 ) and the combined digital signal ( 136 ).

TECHNICAL FIELD

The invention relates to a digital amplifier, a method of providing adesired electrical output power by a digital amplifier and a computerprogram product.

BACKGROUND AND RELATED ART

Magnetic resonance imaging (MRI) is a state of the art imagingtechnology which allows cross sectional viewing of objects like thehuman body with unprecedented tissue contrast. MRI is based on theprinciples of nuclear magnetic resonance (NMR), a spectroscopictechnique used by scientists to obtain microscopic chemical and physicalinformation about molecules. The basis of both NMR and MRI is the fact,that atomic nuclei with none zero spin have a magnetic moment. Inmedical imaging, usually nuclear hydrogen atoms are studied since theyare present in the body in high concentrations for example water. Thenuclear spin of elementary particles can resonate at a resonancefrequency, if a strong DC magnetic field (B₀ field) is applied. Themagnetic resonance (MR) frequency is determined by the level of themagnetic flux. In an MRI scanner, the magnetic field matches a selectedresonance frequency only at one position in space. By varying thisposition step by step, an image can be measured.

The needed strong DC magnetic field is typically generated bysuperconducting magnets. In order to vary these fields, such that itmatches a given radio frequency only at one position, a field gradientis generated using gradient coils. Thereby, the field gradient can varyover time to achieve a scan. The frequency range in the gradient coilsis low and reaches up to a maximum of 10 kHz.

Typically, in MRI apparatus the gradient coils are connected torespective gradient amplifiers. The gradient coils are driven byelectrical currents of several hundreds of Amperes, which need to beaccurately controlled in the range of mA in order to assure anacquisition of MRI images at high quality and precision.

This requires an accurate control of the gradient amplifier output whichcan be for example performed by control circuits using feedback loops.

For example U.S. Pat. No. 6,285,304 B1 discloses an analogue-to-digitalconverter circuit and control device for a gradient amplifier of amagnetic resonance imaging system.

The major disadvantage with such kind of control devices for gradientamplifiers is that these control devices require high precisionelectronics, like high precision digital-to-analogue converters. Thus,the requirements of high precision, resolution and stability makes itimpossible to use commercial analogue-to-digital converters ordigital-to-analogue converters to provide a full digital controlledgradient amplifier for MRI applications.

SUMMARY OF THE INVENTION

The present invention provides a digital amplifier for providing adesired electrical output power, the amplifier comprising a power sourcefor generating the electrical output power, the amplifier furthercomprising a digital input adapted for receiving a digital input signal,the digital input signal representing the desired electrical outputpower level, and a reference power generator for generating an analoguereference power controlled by a digital input signal. The amplifierfurther comprises a power measurement component adapted for measuringthe power differential between the electrical output power provided bythe power source and the analogue reference power. Further, ananalogue-to-digital converter is provided which is adapted forconverting the power differential into a digital power differentialvalue. Further, the amplifier comprises a combiner adapted for providinga combined digital signal by adding a digital power differential valueto the digital value input to the reference power generator forgenerating the analogue reference power, wherein the power source isadapted for providing the electrical power corrected for the powerdifference indicated by the digital input signal and the combineddigital signal.

Embodiments according to the invention have the advantage, that in orderto guarantee a high precision control of the amplifier output,electronic components with larger tolerances regarding their precisioncan be used instead of expensive high-precision electrical components.Existing amplifier designs typically require high precisionanalogue-to-digital converters or high precision digital-to-analogueconverters. A number of digital filter techniques exist to increase theresolution of these converters, which however comes at the cost of theintroduction of considerable dead time of such amplifiers.

The analogue-to-digital converter of the digital amplifier according tothe invention does only need to convert the difference between theoutput signal and the analogue reference power. In other words, theanalogue reference power reduces the dynamic range of the output signalto a lower dynamic range for the feedback signal, such that theanalogue-to-digital converter used for converting the power differentialinto a digital power differential value does not have to be a highprecision analogue-to-digital converter: due to the reduced dynamicrange of the power differential the precision requirements for thisanalogue-to-digital converter can be eased.

In accordance with an embodiment of the invention, the amplifier furthercomprises a feedforward controller adapted for digitally controlling thereference power generator by emulating the output power generationcharacteristics of the power source. In other words, the feedforwardcontroller is preferably receiving the digital input signal representingthe desired electrical output level and provides the reference powergenerator with a modified digital feedforward signal which representsthe amplification behavior of the power source. Preferably, thefeedforward controller is able to emulate at high precision the dynamicamplification behavior of the power source. The usage of a feedforwardcontroller which is able to precisely emulate the power source has theadvantage, that the dynamic range of the power differential provided tothe analogue-to-digital converter is further reduced, which furthereases the restrictions regarding the precision of thisanalogue-to-digital converter in order to provide a precise digitizedpower differential value. Nevertheless, the invention can also becarried out without precisely emulating the amplification behavior ofthe power converter.

In accordance with an embodiment of the invention, the feedforwardcontroller is adapted for emulating the output power generationcharacteristics of the power source corrected for time delay differencesbetween the digital input signal and the digital power differentialsignal. In another embodiment, the difference between the digital inputsignal and the combined digital signal is provided from the power sourceto the feedforward controller. This allows the feedforward controller toself-correct for example for constant errors like offsets in the powersource amplification behavior.

A further possibility to provide a high precision emulation of theoutput power generation characteristics of the power source is that thefeedforward controller is further adapted for performing the emulationof the output power generation characteristics of the power sourcedepending on operational parameters of the power source, wherein theoperational parameters being selected from the group of power sourcecomponent temperature and power source component age. In other words,the feedforward controller comprises an input on which it receives theseoperational parameters of the power source. Aging of power sourcecomponents or temperature changes of individual power source electroniccomponents may change the output power generation characteristics of thepower source in a predictable manner, such that even over a longerperiod of time the feedforward controller is able to precisely emulatethe power source output power generation characteristics.

In accordance with a further embodiment of the invention, the referencepower generator comprises a set of switches digitally controlled bydigital input signal, wherein each switch is controlling the electricaloutput of an amplifier, wherein the reference power generator is adaptedfor setting the level of the analogue reference power by combining theelectrical output of the switched amplifiers. Preferably, the set ofswitches are digitally controlled by the digital output signal of afeedforward controller.

The usage of such switches controlling an amplifier, wherein the levelof analogue reference power is set by combining the electrical output ofthese switched amplifiers is, that a precise reference voltage can beamplified by simple amplifiers, wherein depending on the state of theswitches the amplified signals can be connected to an analogue summator,like for example a precision operational amplifier. In other words,instead of using a rather expensive and complicated high power referencepower generator which is able to simulate an output current which mayhave amplitudes up to 1,000 amperes, simple low power amplifiers can beused which nevertheless provide for high precision and high resolutionreference power generation. It has to be noted here, that even though itis desired to reduce the dynamic range of the power differential signalas much as possible, even in case the analogue reference power is notprecisely emulating the electrical output power provided by the powersource, the precision requirements regarding the analogue-to-digitalconverter are only that the analogue-to-digital converter is able toprecisely digitize variations in the power differential which is at mostin the range of a few hundred mA with a precision of a few mA which ispossible with many state of the art analogue-to-digital converterscommercially available today.

In accordance with a further embodiment of the invention, the powermeasurement component comprises an output magnetic field generatingcomponent adapted for inductively generating a magnetic field from theelectric output power. The power measurement component further comprisesa reference magnetic field generating component adapted for inductivelygenerating a magnetic field from the analogue reference power, whereinthe generated reference magnetic field is directed in opposite directionto the output magnetic field. Further, the power measurement componentcomprises a magnetic field detection component adapted for determiningthe power differential between the electrical output power and theanalogue reference power by measuring a superposition of the outputmagnetic field and the reference magnetic field.

The usage of such kind of power measurement components has theadvantage, that the power differential between the electrical outputpower provided by the power source and the analogue reference power canbe measured without the need of using additional subtractors subtractingthe analogue reference power from the electrical output power typicallydetected by means of a current sensor. In other words, the subtractionof the output signal and the analogue reference power signal is directlyperformed in the power measurement component by means of thesuperposition of the magnetic fields generated by the electrical outputpower and the analogue reference power. The current sensor or magneticfield detection component measures the resulting total magnetic fieldproduced by the electrical output current and reference power currentsrunning through such kind of sensor.

The output magnetic field generating component and the referencemagnetic field generating component may for example just be a singlewire fed with the electrical output power and the analogue referencepower, respectively, wherein these wires are arranged in parallel andwherein the electrical output power current is running in oppositedirection to the electrical current of the analogue reference power.Alternatively, the output magnetic field generating component and/or thereference magnetic field generating component may comprise coils fed inopposite directions by the electrical output current and/or theelectrical reference current.

The magnetic field detection component may also be a pick up coil, or aHall sensor or even a SQUID, which however requires extensive cooling.Nevertheless, since MRI systems typically comprise high power cryogeniccooling systems, such a cooling is feasible.

It has to be noted here, that the invention is not only restricted toMRI systems but can also be applied to any kind of digital amplifierswhich require a high precision feedback control of the output power.This includes amplifier for usage in high precision servo applicationslike for example used in manufacturing facilities for wafer steppers, orother kind of high precision material manufacturing techniques.

In accordance with an embodiment of the invention, the referencemagnetic field creating component comprises multiple coil winding sets,wherein each coil winding set comprises at least one coil winding,wherein the reference power generator is adapted for generating thereference magnetic field level by selecting a number of the coil windingsets for feeding a reference current through the coil winding sets,wherein the number of selected coil winding sets is determined from theanalogue reference power level. In other words, the reference current isgenerated preferably by a single reference current source. By simplyselecting the number of selected coil winding sets, the referencemagnetic field can be generated and controlled by increasing ordecreasing the number of selected coil windings. A high power referencecurrent source is thus not required while it is nevertheless possible toemulate the power source electrical output power generationcharacteristics.

In another aspect, the invention relates to a method of providing adesired electrical output power by a digital amplifier, the amplifiercomprising a power source for generating the electrical output power,wherein the method comprises receiving a digital input signal by theamplifier, the digital input signal representing the desired electricaloutput power level. The method further comprises generating by areference power generator an analogue reference power, wherein thereference power generation is controlled by a digital input signal. Thepower differential is measured between the electrical output powerprovided by the power source and the analogue reference power by a powermeasurement component. Then, the power differential is converted into adigital power differential value by an analogue-to-digital converter. Bya combiner, a combined digital signal is provided by adding the digitalpower differential value to the digital value input to the referencepower generator for generating the analogue reference power, wherein thepower source is providing the analogue reference power corrected for thedifference between the power indicated by the digital input signal andthe combined digital signal.

In accordance with an embodiment of the invention, the method furthercomprises digitally controlling by a feedforward controller thereference power generator by emulating the output power generationcharacteristics of the power source.

In accordance with an embodiment of the invention, the digitalcontrolling of the reference power generator further comprises emulatingthe output power generation characteristics of the power sourcecorrected for time delay differences between a digital input signal anda digital power differential value signal.

In accordance with an embodiment of the invention, the emulation of theoutput power generation characteristics of the power source iscontrolled by the difference between the digital input signal and thecombined digital signal.

In accordance with an embodiment of the invention, the emulation of theoutput power generation characteristics of the power source is performeddepending on operational parameters of the power source, the operationalparameters being selected from the group of power source componenttemperature and power source component age.

In accordance with an embodiment of the invention, the reference powergenerator comprises a set of switches digitally controlled by thedigital input signal, wherein each switch is controlling an amplifier,wherein the level of the analogue reference power is set by combiningthe electrical output of the switched amplifiers.

In accordance with a further embodiment of the invention, the powermeasurement component comprises an output magnetic field generatingcomponent, a reference magnetic field generating component and amagnetic field detection component, wherein the method further comprisesinductively generating by the output magnetic field generating componentan output magnetic field from the electrical output power andinductively generating by the reference magnetic field generatingcomponent a magnetic field from the analogue reference power, whereinthe generated reference magnetic field is directed in opposite directionto the output magnetic field. The method further comprises determiningby the magnetic field detection component the power differential betweenthe electrical output power and the analogue reference power bymeasuring a superposition of the output magnetic field and the referencemagnetic field.

In accordance with a further embodiment of the invention, the referencemagnetic field creating component comprises multiple coil winding sets,wherein each coil winding set comprises at least one coil winding,wherein the method further comprises generating by the reference powergenerator the reference magnetic field level by selecting a number ofthe coil winding sets for feeding a reference current through the coilwinding sets, wherein the number of selected coil winding sets isdetermined from the analogue reference power level.

In another aspect, the invention relates to a computer program productcomprising computer executable instructions to perform any of the methodsteps according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention are described ingreater detail by way of example only making reference to the drawingsin which:

FIG. 1 is a block diagram illustrating an amplifier according to theinvention,

FIG. 2 is a further block diagram illustrating an amplifier comprisingswitched amplifiers as reference power source,

FIG. 3 is a block diagram illustrating a current sensor for an amplifieraccording to the invention,

FIG. 4 is a flowchart illustrating the method of operating an amplifieraccording to the invention.

DETAILED DESCRIPTION

In the following, similar elements are depicted by the same referencenumerals.

FIG. 1 is a block diagram illustrating a digital amplifier according tothe invention. The digital amplifier comprises a power source 100 with adigital input adapted for receiving a digital input signal 112. Thisinput signal is a predetermined current curve shape that is availabledigitally at a resolution of k-bit. The output signal 140 of the powersource 100 is a current flowing for example through a gradient coil, orin general a load 110. Ideally, the output signal 140 has the same curveshape as the input signal 112. The output current can have amplitudes upto 1,800 A, which however is not limited to this value, while themaximum deviation of the ideal curve shape is only allowed to be a fewmA.

The power source 100 comprises a subtractor 102, a digital controller104, a modulator 106 and a power converter 108. The output signal 140 isgenerated by the power converter 108 which converts a low power pulsewidth modulated (PWM) signal to a high power signal that drives the load110. Most common is that the power converter 108 puts a high outputvoltage to the inductive load 110, resulting in a high current throughthe load, wherein this current through the load is called the outputsignal.

The modulator 106 is usually and preferably a digital device thatconverts the control signal 116 received from the digital controller 104into a PWM signal 118. This PWM signal 118 is provided to the powerconverter 108.

The digital controller 104 reads the error signal 114 between the inputsignal 112 and a realized output signal 140 and provides appropriatecontrol signals to the modulator 106 to counter the error signal.Preferably, the input signal 112, the error signal 114 and the controlsignal 116 are digital signals with a resolution of k-bits.

In order to determine the error signal between the input signal 112 andthe realized output signal 140, the output signal 140 needs to bedetected and analyzed. Detection is performed by means of the powermeasurement component 142. Analysis of the detected output signal withrespect to the input signal 112 is performed by a feedforward and afeedback loop.

A feedforward controller 120 reads the input signal 112 and predicts theoutput signal as accurate as possible based on a model of the system.For example, the feedforward signal is corrected for time delaydifferences between the input signal, the feedback signal 132 in thefeedback loop and the feedforward signal 122 supplied from thefeedforward controller 120 to a bias generator 124 adapting forgenerating a reference current. In other words, the bias generator 124converts the digital feedforward signal 122 into an analogue signal 126to bias the output signal 140.

The feedforward controller 120 may further be adapted for receivingoperational parameters of the power source 100. For example, the powerconverter 108 may comprise several temperature sensors which temperaturevalues are provided preferably digitally to the feedforward controller120, which in turn may correct its used model of the system. Further,the feedforward controller 120 may be adapted to receive error signals114 from the power source 100 such that the feedforward controller 120is able to self-correct for example for constant errors like offsets inthe output signal 140. The origin of the error signals 114 is describedin detail below.

The digital amplifier illustrated in FIG. 1 further comprises asubtractor 128 which subtracts the bias signal 126 from the outputsignal 140 detected by the power measurement component 142. Thissubtraction results in an analogue power differential current which isprovided to the analogue-to-digital converter 130.

The analogue-to-digital converter 130 converts the difference betweenthe output signal 140 and the bias signal 126 from the analogue to thedigital domain. This analogue-to-digital converter (ADC) is required toconvert only the difference between the output signal and the predictedoutput signal. Thus, the bias signal reduces the dynamic range of theoutput signal (k-bits) to a lower dynamic range for the feedback signal(m-bits).

The feedback signal 132 is combined by means of a combiner 134 with thefeedforward signal 122. This results in a determined output signal 136which is provided to the power source 100. For example, the power source100 comprises a subtractor 102 which subtracts the determined outputsignal 136 from the input signal 112. This results in the error signal114 which is fed to the digital controller 104 as already describedabove.

Thus, the feedforward signal 122 is a digital representation of thepredicted output signal. The feedback signal 132 is a digitalrepresentation of the difference between the measured output signal andthe predicted output signal. The combination of the feedforward 122 andfeedback signal 132 is the digital representation of the measured outputsignal, called determined output signal 136. The difference between thedetermined output signal and the input signal is the error signal 114.

Even though, the bias signal 126 may not predict the output signal 140at maximum accuracy, the dynamic range of the difference between theoutput signal 140 and the bias signal 126 is typically less than 1% ofthe output signal 140. As a consequence, the dynamic range of thisdifference in output signal and bias signal is rather low, such that asimple AD converter 130 is only required in order to accurately digitizethe difference between the output signal and the bias signal forreceiving the feedback signal 132 with a resolution of a few mA.

In order to illustrate this in more detail, the input signal 112 is adigital signal with a resolution of k-bit. The feedforward signal 122 isa digital signal with a resolution of n-bit, wherein n<k. The feedbacksignal 132 is a digital signal with a resolution of m-bit, wherein m<kand n+m=k. The m least significant bits of the determined output signalare a representation of the feedback signal, wherein the n mostsignificant bits of the determined output signal is a representation ofthe feedforward signal.

It is for example expected that the feedforward controller 120 canproduce a feedforward signal of 6 bits (n=6). This allows using a simplefeedforward controller to predict the output signal with moderateaccuracy meeting the requirement of 1% mentioned above. It is furtherassumed that the input signal has a resolution of 18 bits (k=18). Therequired resolution of the AD converter 130 is m-bit (m=k−1=12 bits).This AD conversion can be realized by a 12 bit AD converter but can alsobe realized by an ADC with less bits in combination with resolutionenhancement techniques like over-sampling.

FIG. 2 illustrates a part of the digital amplifier of FIG. 1, with thereference power generator (or bias generator) 124 being represented by aset of switches 210, 212 and 214, a set of amplifiers 202-206 and ananalogue summator 208. In the embodiment depicted in FIG. 2, thefeedforward signal 122 which is a digital signal of n-bits controls nswitches 210, 212 and 214 that set the bias voltage. A precise referencevoltage from a source 200 is amplified by n-amplifiers 202, 204 and 206by a factor 2⁰, 2¹, . . . , 2^(n). Depending on the state of theswitches 210, 212 and 214, these amplified signals can be connected toan analogue summator, like for example a precision operational amplifier208. The output of the summator 208 is the bias voltage or bias signal126 which is subtracted from the current sensor 142 output voltage. Thisresults in the analogue feedback signal which represents the differencebetween the output current and the bias signal. The analogue feedbacksignal is then digitized by the AD converter 130 resulting in thefeedback signal 132.

In FIG. 2, the output signal is measured by an accurate current sensor142. Normally such a sensor generates a current that is a scaledrepresentation of the output current through the gradient coil, or ingeneral the load 110. Also, the sensor output is preferably galvanicallyisolated from the power converter 108 and the load 110. The sensoroutput current is converted to a voltage by means of for example aburden resistor.

However, it has to be noted here that the current sensor may be any kindof state of the art current sensors, including Hall sensors, SQUIDsensors, sensors working with inductively coupled coils etc.

In the embodiment depicted in FIG. 2, only one accurate reference source200 is required, wherein further the components like the operationalamplifiers 202-206 and the summator 208 can be implemented by simple andcommercially available components. Nevertheless, it should be noted,that the amplifiers 202-206 used to amplify the individual bias voltagesshould also be rather accurate. In order to further improve thisaccuracy, it is preferred to compensate for static gain errors of theseamplifiers by for example calibration.

FIG. 3 illustrates a further embodiment of a digital amplifier accordingto the invention. In contrast to the power measurement componentconsisting in FIG. 2 of the sensor 142 and the subtractor 128, the powermeasurement component in FIG. 3 is given by a combination of a pickupcoil 310 and several conductors 302-308. In the embodiment of FIG. 3, aseparate subtractor 128 is not required anymore.

Assuming, that the conductors 302-308 are coil windings located forexample within a pickup coil 310, the output of the power converter 108may be connected by one coil winding 302 to the load 110. The biascurrents flowing through the windings 304-308 are generated by a singlereference current source 301. Depending on the state of the switches300, set by the feedforward signal 122 by means of a control signal 312,the reference current (bias current) flows through a number of windings304-308 in the current sensor 310. The windings 304-308 through thecurrent sensor representing the bias signal of the individual bits ofthe feedforward signal have a number of turns related to the bitposition of the feedforward signal 122. Because the output current is upto 1,000 A (not limited to, see earlier) and normally measured by asingle turn in the current sensor 310, either the bias current should belarge or the number of bias turns 304-308 should be relatively high. InFIG. 3, the number of turns is shown as a ·2^(n). Hereby, a is aninteger number.

Important in FIG. 3 is, that by means of the coil windings 304-308 ofthe pickup coil 310 and the coil winding 302 through which the outputcurrent from the power converter 108 to the load 110 is flowing,resulting magnetic fields are generated which point in oppositedirections. This can be achieved in two ways: 1) having the current flowin opposite directions, 2) having the windings through the core inopposite directions. FIG. 3 shows opposite direction of windings throughthe core, so the current should be have same sign. This results in thegeneration of two opposite magnetic fields.

As a consequence, the current sensor output 312 detected by the sensor310 ‘automatically’ only comprises the difference between the outputsignal and the predicted reference output signal. Thus, the dynamicrange of the current sensor output 312 is already reduced such that theAD converter 130 is only required to convert the analogue current sensoroutput signal 312 to the digital feedback signal 132 with a resolutionof m-bit, wherein the input signal is a digital signal with a resolutionof k-bit, the feedforward signal is a digital signal with a resolutionof m-bit with n<k and with m<k and n+m=k.

FIG. 4 is a flowchart illustrating the method according to the inventionof providing a desired electrical output power by a digital amplifier.In step 400, a digital set point of resolution k-bits is received. Apower source is generating in step 402 the output signal specified bythe digital set point. Further, in step 404 a feedforward signal isgenerated by a feedforward controller, wherein the generated feedforwardsignal has a resolution of n-bits, wherein n<k. From the feedforwardsignal, in step 406 an analogue bias signal is generated, wherein instep 408 an analogue power differential between the analogue bias signaland the generated analogue output signal is determined.

In step 410, this analogue power differential is converted into adigital feedback signal, wherein the feedback signal is a digital signalwith a resolution of m-bit. In step 412, a determined output signal iscalculated by combining the feedforward signal and the feedback signal.From the determined output signal, in step 414 an error signal isdetermined describing the difference between the desired electricaloutput level specified by the digital input signal and the measuredanalogue output signal. This is followed by step 416 in which the actualoutput signal is corrected by the error signal for providing a correctedoutput signal in step 416.

REFERENCE NUMERALS

-   100 Power source-   102 subtractor-   104 digital controller-   106 modulator-   108 power converter-   110 load-   112 input signal-   114 error signal-   116 control signal-   118 PWM signal-   120 feedforward controller-   122 feedforward signal-   124 bias generator-   126 bias signal-   128 subtractor-   130 AD converter-   132 feedback signal-   134 combiner-   136 determined output signal-   140 output signal-   142 sensor-   200 voltage source-   202 amplifier-   204 amplifier-   206 amplifier-   208 summator-   210 switch-   212 switch-   214 switch-   300 switch-   301 reference current source-   302 winding-   304 winding-   306 winding-   308 winding-   310 pickup coil-   312 control signal

1. A digital amplifier for providing a desired electrical output power,the amplifier comprising a power source for generating the electricaloutput power, the amplifier further comprising: a digital input adaptedfor receiving a digital input signal, the digital input signalrepresenting the desired electrical output power level, a referencepower generator for generating an analogue reference power controlled bythe digital input signal, a power measurement component adapted formeasuring the power differential between the electrical output powerprovided by the power source and the analogue reference power, ananalogue-to-digital converter adapted for converting the powerdifferential into a digital power differential value, a combiner adaptedfor providing a combined digital signal by adding the digital powerdifferential value to the digital value input to the reference powergenerator for generating the analogue reference power, wherein the powersource is adapted for providing the electrical power corrected for thedifference between the power indicated by the digital input signal andthe combined digital signal.
 2. The digital amplifier of claim 1,further comprising a feedforward controller adapted for digitallycontrolling the reference power generator by emulating the output powergeneration characteristics of the power source.
 3. The digital amplifierof claim 2, wherein the feedforward controller is further adapted foremulating the output power generation characteristics of the powersource corrected for time delay differences between the digital inputsignal and the digital power differential value signal.
 4. The digitalamplifier of claim 2, wherein the emulation of the output powergeneration characteristics of the power source is controlled by thedifference between the digital input signal and the combined digitalsignal.
 5. The digital amplifier of claim 2, wherein the feedforwardcontroller is further adapted for performing the emulation of the outputpower generation characteristics of the power source depending onoperational parameters of the power source, the operational parametersbeing selected from the group of a power source component temperatureand power source component age.
 6. The digital amplifier of claim 1,wherein the reference power generator comprises a set of switchesdigitally controlled by the digital input signal, wherein each switch iscontrolling the electrical output of an amplifier, wherein the referencepower generator is adapted for setting the level of the analoguereference power by combining the electrical output of the switchedamplifiers.
 7. The digital amplifier of claim 1 wherein the powermeasurement component comprises: an output magnetic field generatingcomponent adapted for inductively generating a magnetic field from theelectrical output power, a reference magnetic field generating componentadapted for inductively generating a magnetic field from the analoguereference power, wherein the generated reference magnetic field isdirected in opposite direction to the output magnetic field, a magneticfield detection component adapted for determining the power differentialbetween the electrical output power and the analogue reference power bymeasuring a superposition of the output magnetic field and the referencemagnetic field.
 8. The digital amplifier of any of claim 7, wherein thereference magnetic field creating component comprises multiple coilwinding sets, each coil winding set comprising at least one coilwinding, wherein the reference power generator is adapted for generatingthe reference magnetic field level by selecting a number of the coilwinding sets for feeding a reference current through the coil windingsets, wherein the number of selected coil winding sets is determinedfrom the analogue reference power level.
 9. The digital amplifieraccording to claim 1 wherein the amplifier is a gradient amplifier of amagnetic resonance imaging system.
 10. A method of providing a desiredelectrical output power by a digital amplifier, the amplifier comprisinga power source for generating the electrical output power, the methodcomprising: receiving a digital input signal by the amplifier, thedigital input signal representing the desired electrical output powerlevel, generating by a reference power generator an analogue referencepower, wherein the reference power generation is controlled by thedigital input signal, measuring the power differential between theelectrical output power provided by the power source and the analoguereference power by a power measurement component, converting the powerdifferential into a digital power differential value by ananalogue-to-digital converter, by a combiner, providing a combineddigital signal by adding the digital power differential value to thedigital value input to the reference power generator for generating theanalogue reference power, wherein the power source is providing theelectrical power corrected for the difference between the powerindicated by the digital input signal and the combined digital signal.11. The method of claim 10, further comprising digitally controlling bya feedforward controller the reference power generator by emulating theoutput power generation characteristics of the power source.
 12. Acomputer program product comprising computer executable instructions toperform any of the method steps as claimed in claim 10.